Microscopic model for the formation of nanodomains in relaxor materials

A microscopic relaxor model is presented and its properties are studied by means of numerical simulations. No ``a priori'' random interactions or random fields are considered. The sole interaction of mobile charges with the dipoles of a highly polarizable medium is responsible for the appearance of typical relaxor properties such as a smeared transition, a glassy ground state with no macroscopic polarization, an external field frequency-dependent dielectric constant, and a field-induced ferroelectric behavior. The model shows no statistical difference between the relaxor and the ferroelectric formation of polar nanoregions at high temperature. However, for the relaxor, there is a temperature (which we associate with the Burns temperature) below which some dipoles, neighbors to the charges, get frozen with a net square polarization different from zero. These dipoles are the precursors of stable boundaries from which ferroelectric clusters grow when approaching the glassy phase. These frozen dipoles are responsible for the frequency dispersion of the dielectric constant and for the smearing of the transition.